Wireless local area network (WLAN) products based on the Institute of Electrical and Electronics Engineers' (IEEE) 802.11 standards are often referred to as Wi-Fi technology. All devices that are connected via a WLAN are referred to as stations or network stations. All network stations are equipped with wireless network interface controllers (WNICs). Network stations are either access points or clients. Access points (APs), typically routers, are base stations for a WLAN. Clients are devices such as personal computers, video game consoles, smartphones, tablet computers, digital audio players and wireless sensor devices.
WLAN operating according to the IEEE 802.11 standards use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) that attempts to avoid collisions between transmissions. Under CSMA/CA, when a device needs to transmit, it listens to the network (senses the carrier) and waits for it to be free before beginning a transmission.
When two WLANs are in close proximity, transmission interference between the two WLANs can be avoided by each transmitting on different channels. For example, IEEE 802.11a operates in the 5 GHz band and has 22 or 23 non-overlapping channels. IEEE 802.11g is divided into 13 channels spaced 5 MHz apart, with channel 1 centered on 2.412 GHz and channel 13 centered on 2.472 GHz. IEEE 802.11g uses 20 MHz width signals allowing for four non-overlapping channels. IEEE 802.11n operates both on the 2.4 GHz band and on the 5 GHz band. Availability of channels is regulated by country. In North America, for example, only channels 1 through 11 are currently allowed to be used.
The IEEE 802.11a/g/n standards use orthogonal frequency-division multiplexing (OFDM) and define a preamble used to acquire frequency, phase and symbol timing. The beginning of this preamble consists of a sequence of eight repeated symbols used for, among other things, coarse frequency acquisition. The symbols transmitted during the beginning of the preamble employ fewer subcarriers than are used during normal data transmission in order to make for wider subcarrier spacing in order to allow the receiving station to acquire frequency lock over a wider range of frequency offsets between transmitting station and receiving station.